Electrically heated glass panes are known to the prior art. A fairly representative prior art teaching of a glass pane of this type may be found in German Offenlegungsschrift No. 16 90 298.
The German publication discloses a glass pane including a pair of power supply conductors and a plurality of heating conductors connected at their opposite ends to the power supply conductors. Each of the heating conductors is characterized by a relatively narrow band. The German publication, also, discloses a cover which serves as a protective layer for the conductive layer therebelow. The cover layer is applied as a printable paste on each of the conductors previously pressed-on and dried, and both of the layers are burnt-in in the surface of the glass pane in a single heat treatment operation.
According to the German publication the heating conductors are of substantially constant cross section throughout their length, even at the point of transition or junction with the power supply conductors.
It has been recognized that the protective layer, if it is to provide the desired function, normally extends laterally of the power supply conductors of the conductive layer. Thus, not only are the power supply conductors covered, but, also, the points of transition of the heating conductors into the power supply conductors are covered as well. Accordingly, these points of transition no longer are accessible from the outside. And, weak points in connection of the several heating conductors and power supply conductors which may result from the printing process are not correctable. A resultant problem is the possible breakdown and interruption of the electric path at these weak points through localized overheating.
In case of electrically heated glass panes, where the heating conductors are provided with a galvanically applied metal, reinforcing layer, the protective layer may only be applied prior to the galvanizing process and the galvanically applied layer only may reach to the limiting edges of the protective layer. This factor may be tolerated if it becomes necessary to apply a protective layer on the power supply conductors. Thus, a protective layer is necessary when the power supply conductors are disposed close to the edge of the glass pane, and particularly in the region serving as an edging of the glass pane in the frame of the body, when the glass pane is attached to the frame by an adhesive. To this end, the adhesive masses or primary layers for enhancement of adhesion oftentimes have an unfavorable influence on power supply conductors. The protective layer, as set out, prevents access to the points of connection of the heating conductors and power supply conductors with the consequences as set out, also. In case of electrically heated glass panes having both galvanically reinforced heating conductors and a protective layer over the power supply conductors, as well as laterally of the power supply conductors, the problem of possible weak places in the transition area between heating conductors and power supply conductors is compounded. This is because the cross section of the heating conductors, between the limiting edges of the protective layer, is increased by the metal, reinforcing layer.